Proof: Metal Gear Solid Peace Walker Is Kinda MGS5.

It has no "number". But apparently there was a time when the game did. Look at this logo. It's an old one that was posted on Kojima Productions' developers blog by Jiro Oishi. It says "Metal Gear Solid 5" on it.

As we all know, the number was dropped before the E3 reveal. But if you ever wanted more proof that - spiritually at least, Peace Walker is MGS5 - well, there it is.